Method of firing porcelain



April 3, 1951 BEAUDET METHOD OF' FIRING PORCELAIN Filed April 15, 1947Patented Apr. 3, 1951 UNITED STATES ATEN i'OFFlCE A METHOD .OF FIRINGPORCELAIN Louis Beaudet, Limoges, France, assignor to Society Bcrnardaud& Cie, Limoges, France Application April 15, 1947, Serial No. 741,651 InFrance April 17, 1946 locat s. (01.25-45 7 My invention has for itsobject a method for such an operation; that carries on for more than tenhours, can be imagined. The loading and deslagging bring insubstantialmodifications in the composition of the fumes, and consequently of theatmosphere surrounding the various products. This atmosphere can changefrom reduc ing to carburising. if the amount of air coming in The piecesfirst of all are biscuit baked or semi-vitrified and then enamelled, andafter this they are subjected to the final burning that is carried outby a gradually increasing rise in temperature. This burning is executedin three phases.

The first phase takes place in an oxidising atmosphere up to the coatingtemperature that varies from 800 to 1100" 0.; this oxidising atmospherewas regarded as quite necessary to enisnot enough to ensure theconversion of the carbon in the fuel to carbon monoxide. It may changeto oxidising if there is a lack of fuel at some part of the grate or anuneven distribution on it. These irregularities bring in numerousdefects such as yellow or smoked impression, granulations, blisters orbubbles, dark edges, etc. all shortcomings that involve .a discharge tothe waste-dumpof quite a substantial part of the production."

sure the burning of the carbon-contents in the biscuit paste or in theenamel and for avoiding any carbon deposits in the paste and enamel as aresult of the dissociation of the gases resulting from the burning anddistillation of the fuel.

The second phase takes place in a reducing atmosphere and is intended toensure the reduction of the salts or the colouring metallic oxidesoccurring in the paste, especially iron compounds. This atmosphere iseither kept stable until the end of firing, or only until the reachingof a temperature lower than that denoting the finish of the burning.This temperature at which the action of the reducing atmospherestops isoften in relation with the requirements of the baking of the raw pasteas semi-vitrified when this-is done by recovery of the fumes ofanintermittent furnace.

If required the baking ends in an atmosphere which is neutral orslightly oxidising, often beon account ofthe recovery executed asmentioned above.

The burning is executed either in an intermittent or in a continuouslyoperating furnace.

. causeof the'manuiacturers personal reasons or Intermittent furnacesare made as a .rule with two chambers of which one receives thesemivltrified and enamelled pieces that are subjected therein to thefinal burning. The other chamber warmed by the fumes discharged from thefirst chamber, takes the pieces of raw non-enamelled paste that arebaked therein to form semivitrified or biscuit pieces.

The transition from the oxidising to the reducing atmosphereis effectedin ,a hearth with v solid fueLby damping the fire so as to cause the a tt rk a a a -ge r t r: 10?. em rator, a secondary air-inlet allows-thepartial burning of the gases coming from'the distillation andgasification of the fuel. The trouble with With intermittent furnacesusing liquid fuel the same drawbacks occur in spite of their apparentstraightforwardness.'

In the straight-flow furnaces, very often of tunnel-type the methodspractised vary according to the manner of heating.

When the heating is done with a volatile gas, the burners, located inthe high temperature area, are regulated so as to form reducing fumeswith a certain content of unburnt or partly burnt gas, expressed as anamount of carbon monoxide and hydrogen detected by chemical analysis.However, the very last burners, in the direction of the flow of theproducts, may be regulated as neutral or oxidising in order to producethe third phase mentionedabove.

In the drawing,

Fig. 1 is a diagrammatic view illustrating the novel method of thepresent invention; and

.Fig. 2 is a temperature curve obtained from practicing the method ofthe invention.

Referring to the diagram of Fig. 1,,the fumes traversingthe area CB arereducing. At B, the point at which thetemperature is kept close tocoating point, hotair is injected intothe fumes in .Orderto burn off.the excessof volatile gas and to makej'the atmosphere oxidising in thearea BA. In certain processes, not only is an injection made at B of hotair, but also the reducing fumes a e drawn off from the same point sothat the area BA may besubjected only to an air flow. The hot air is asarule taken off that formed by the cooling of the products.

The consequence of. this is a vast complication in the furnaceconstruction and great trouble with its regulation and itsstabilization.

Apart from the temperature and the condition of the atmosphere anotherfactor comes into the picture: the duration of the burning and of thecooling The length of time for the rise in tern-' ven ure is. co mon y.5' h r and r. he C001. ing 3 0' hours Therejissometimes a lesserduration' required in the tunnel furnaces. I I In short, with theprevailing arrangements, it

is a matter of difficulty to regulate suitably the ranges of atmospheresthat must be adjusted in the baking furnace.

The process constitutin the object of the invention aims at a cure ofthe drawbacks of the previous processes.

It consists in that the two main phases of the final burning that hasbeen the practice up to the present as stated above, the first in anoxidising and the second in a reducing atmosphere, are replaced by asingle phase in a reducing atmosphere.

The burning can be finished off as in the known processes by a shortneutral or slightly oxidising period.

The new manner of carrying out the work is founded on the followingpoints:

The carbon contents in the biscuit baked paste I or in the enamel, aswell as that deposited during the different storings and handlings inthe form of dust, grease from workmens hands etc. need not be burnt, inother words there is no necessity to convert it into carbonic gas butonly to gasify same and this can be done at any temperature providedthat at this temperature the paste in the process of baking is porousenough and the enamel not caked by the sintering that takes place beforethe meltin and vitrification. The conversion into gas of the carbon maybe got by any of the following well-known reactions:

These reactions are characteristic of chemical balances and when C, C0,C02, H and H2 are present, a complex balance is set up that depends onpressure, temperature, the concentration of the elements present and-onthe length of contact.

Theoretically speaking, for a sufliciently long contact period, above700 C. the balance has a tendency to separate out carbon as given out bythe equation 2CO=CO2:-C.

But the time required for reaching the balance is far above the timeallowed for contact in varying atmospheres.

a tunnel furnace, so that there is little need to fear a carbon deposit.From 750 C. onwards the balance leads to the entire disappearance ofcarbon, as such by the reaction of carbon in carbonic acid as by thereaction on steam.

The further the temperature rises and the higher the speed of reactionincreases, so that at 1000 C. the disappearance of the carbon ispractically instantaneous and it becomes impossible to have any freecarbon in the presence of carbonic acid and steam.

The consequence of this is that the heating period in an oxidisingatmosphere is quite useless, and one of the main features of the newprocess is to burn in reducin atmosphere from the beginning to the end,except finally a short terminal period in neutral or slightly oxidisingatmosphere.

In the case of heating by burning a gas, the fumes contain amounts ofcarbonic acid and steam substantially over those strictly required forthe conversion of carbon into gas. The removal of the carbon is thenautomatic and the amounts of carbon monoxide thus formed are not sogreat that they can have any real effect on the composition of thefumes.

On the other hand, it was granted up to nOW that the reduction of saltsand metallic oxides contained in the paste called for a material amountof reducer gas in the fumes. In fact, the experiments undertaken by theapplicant have proved that a minute percentage is enough a paste with acontent of 3% iron oxide being reduced entirely in 9 hours by anatmosphere carrying 0.4% carbon monoxide and produced by burning towngas. In fact, it is enough then for the very low percentage of iron thatcharacterises the paste used for porcelain and as a rule any ceramicpaste, to have an atmosphere which is very slightly reducing andcontains practically less than 1% of carbon monoxide and hydrogen.

This very low carbon monoxide content helps moreover to a large extentthe conversion of carbon to gas and above all further checks the alreadyminute chances of carbon deposition at low temperature, in the areacorresponding to the breaking up of the carbon monoxide. Thisdissociation is given out in the chemical equation:

The lower the percentage of CO in relation to that of CO2 and the lowerthe temperature from which the dissociation starts and, consequently theslower will be the reaction and thereby, in a given period, thedeposition of carbon. For this reason the atmosphere to be obtained hasas its special feature a reduced percentage of carbon monoxide and araised percentage of carbonic acid, accompanying increased steamcontents being additionally of great interest.

These comments are equally important for the introduction of the processfor furnaces performing the burning without combustion such as electricfurnaces of all kinds or without contact between the products to beburnt and the combustion products or heating fluids.

In this case an artificial atmosphere is kept up in accordance with theprocess requirements throughout the length of the furnace effecting theheating, but there is a possibility of cutting out the arrangements suchas fire doors, blowers or other accessories planned to keep the area athigh temperature separate from that of preheating and in which theformer practice was to have In brief, the improved process possessesoutstanding points in its favour through the simplicity of the method ofbuilding the furnaces. The more or less complicated arrangements usedfor the avoidance of air in the high temperature area, for bringing backto the particular point where the coating temperature of glaze coatingis obtained the hot air recovered in the cooling area or for removing inthe preheating area all contact between the products to be baked and thefumes coming from the high temperature area, become quite useless. Thetroubles met with in regulating the furnace, especially those derivedfrom the uneven balancing of the pressures are substantially lessened.The hot air recovered in the cooling area may be used completely at anyrate for the partial heating of the drying kiln that is generallyassociated with the baking furnace.

Another main feature of the invention relates to the speed of baking andcooling.

The glazing of the hard porcelain paste is done at a temperature below1200 C. and can be executed in less than four hours. Under suchcircumstances, in accordance with the new process, the complete bakingof hard porcelain, with good surface covering of the enamel, takes lessthan nine hours.

What is more, apart from the problems of atmospheres, applicantsinvestigations have proved that the cooling of the porcelain could bedone very quickly. Thus the porcelain baked at 1400" C. may be cooled atthe rate of 700 C. per hour between 1400" C. and 700C. On theother hand,for the quality of the enamel, as much for its colouring as for itssurface appearance, it is far. better to carry out a very slow rate ofcooling for the first 50 C. of cooling, after which it can proceed veryquickly. The baking curve that is obtained from carrying out the newprocess in The arrangements featured are presented only as examples: allthe details of execution and undertaking may be modified in every casewithout widening the scope of the invention as defined in accompanyingclaims.

What I claim is:

1; The method of firing enamelled porcelain consisting in firing theporcelain from the beginning until near the end of the firing operationin a reducing atmosphere including a smal1 percentage of carbonmonoxide.

2. The method of firing enamelled porcelain consisting in firing theporcelain from the beginning until near the end of the firing operationin a reducing atmosphere including a small percentage of carbon monoxideand a higher percentage of carbon dioxide and of steam.

condition, the curve may for a given baking re- 7 sult in the minimumfuel consumption.

The substantial shortening of the baking time is a favourable feature ofthe process, but an increase of the time beyond the necessary minimumdoes not introduce any change.

The practice of the new process when the baking time is greatlyshortened calls for a close contact between the piece andthe furnaceatmosphere. The saggars or boxes of refractory material containing thepieces therefore must be amply notched or put together in extremelyporous material. The packing of the pieces as used at present in roundkilns would not allow a suitable performance of the new process. Infact, as a result of the low porosity of these boxes of refractorymaterial the porcelain bakes in the air enclosed in the box at the timeof the stacking of the pieces while this air is not driven out or onlyvery slowly by the inlet of the reducing atmosphere at the moment of thehardening of the paste. The pieces baked in these conditions, when lessthan six hours pass between the beginning of the baking and the reachingof the temperature of hardening of the paste, when viewed intransparence, are reddish, their surface is yellow and often hasbubbles. Their enamel is waxy;

In accordance with the invention, open boxes are employed as explainedfor ensuring a close contact between the baked pieces'and the furnaceatmosphere.

Another advantageous point of the invention is that the new processallows the baking of the furnace blue or Sevres blue in much bettercircumstances than at present exist. As is known, this blue is obtainedby means of cobalt oxide. This blue changes in very highly reducingatmosphere. On the other hand, cobalt oxide volatilises in an oxidisingatmosphere, starts to deposit on the refractory boxes, and spreads outinto the furnace where it may be deposited on white pieces. In the newprocess, these two drawbacks are avoided and it is possible to bakepieces in blue at the same time and in the same furnace as white pieces.As the volatilization of the colouring agent is altogether cut out, theboxes that have had blue pieces may be used again for white ones.

The new process thus forms an innovation of high interest for the bakingof furnace blue.

The baking of so-called sharp fire enamels may be carried out equallyadvantageously.

3. The method of firing enamelled porcelain consisting in firing theporcelain from the begin- I ning until near the end of the firingoperation in a reducing atmosphere, for a period approaching but notexceeding nine hours.

4. The method of'firing enamelled porcelain consisting-in firing theporcelain from the beginning until near the end of the firing operationin a reducing atmosphere and then cooling the porcelain at a rate of 700C. per hour between 1400 C. and 700 C.

5. The method of firing enamelled porcelain consisting in firing theporcelain from the beginning until near the end of the firing operationin a reducing atmosphere, then cooling the porcelain very slowly andthen cooling the porcelain at a rate of 700 C. per hour between 1400 C.and 700 C.

6. The method of firing porcelain enamelled with furnace blue and thelike sharp fire enamels consisting in firing the porcelain from thebeginning until near the end of the firing operation in a slightlyreducing atmosphere including less carbon monoxide than carbon dioxideand then heating the porcelain for a short time in a substantiallyneutral atmosphere.

7. The method of producing hard porcelain from semi-vitrified enamelledpieces which comprises directly firing said pieces from the beginninguntil near the end of the firing operation in a reducing atmosphere.

8. The method as set forth in claim 7 which comprises in addition, thestep of heating the fired products for a short time in a neutralatmosphere.

9. The method as set forth in claim 7 which comprises in addition, thestep of heating the fired products for a short time in an oxidizingatmosphere.

10. The method of producing hard porcelain from semi-vitrified enamelledpieces which comprises firing said pieces from the beginning until nearthe end of the firing operation in contact with a continually renewedreducing atmosphere.

LOUIS BEAUDET.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,577,124 Jackman Mar. 16, 19261,802,776 Pence Apr. 28, 19 1 1,968,052 Norton et a1. July 31, 19342,112,777 Hauser Mar. 29, 1938

